Pathways to the Abyss

Spiders of the Sea – Pycnogonids

One of the pycnogonids collected in the trawl.Image courtesy of Liz Baird, Deepwater Canyons 2013 Expedition, NOAA-OER/BOEM/USGS

A question from Zach came through the blog recently, asking about the coolest animal we have brought up so far. Pycnogonids, or sea spiders, was the answer, and after learning more about them, an entire post on these extraordinary creatures was due.

These “sea spiders” are found all over the world, from shallow water reefs in the tropics to these deep areas of the canyons. They range in size from about a third of an inch to 35 inches. Although they are arthropods, they aren’t truly spiders, and there is debate where they should be classified. There is some thought that they are an ancient sister group to all living arthropods. Pycnogonid means “thick knees” and they certainly have obvious joints along their legs. When they walk, the resemblance to spiders is unmistakable, however they can also swim by pulsing their legs to propel themselves through the water.

The dark orange proboscis is nearly as long as the yellow thorax of this pycnogonid. The ovigers are bent backwards towards the head, looking like paired loops in this image. Looking closely, you might see the hooks at the end of the ovigers.Image courtesy of Liz Baird, Deepwater Canyons 2013 Expedition, NOAA-OER?BOEM/USGS

Holding one in your hand it is difficult to tell which side is up. The super skinny body and long proboscis (for feeding) don’t provide many clues. However if you look closely you can find two tiny eyes, which indicate the top of the head. The proboscis is used for feeding. The pycnogonid inserts it into animals such as anemones and sucks nutrients out, much like a mosquito sucks blood from humans. The anemone does not die, just as we do not die from a mosquito bite. The proboscis on the specimens we collected is nearly as long at the entire body!

The eyes are located where the dark orange proboscis joins the head. On either side of the proboscis are the chelifores, for feeding and the next appendage out are the ovigers. The yellow legs join the very thin thorax.Image courtesy of Liz Baird, Deepwater Canyons 2013 Expedition, NOAA-OER/BOEM/USGS.

The anatomy of the pycnogonid is very confusing. Most pycnogonids have 4 pairs of walking legs, as do spiders. However there are some species with 5 or 6 pairs of walking legs. Most of us have seen the diagram showing three body parts for many arthropods – head, thorax and abdomen. In pycnogonids the thorax, where the legs attach, is the main “body” and the abdomen is tiny and almost vestigial. This means there is not much room for organs inside! There are several adaptations to this, for example pycnogonids do not have a respiratory system, rather they “breathe” through diffusion. Their long skinny legs and skinny body provide lots of surface area for this process. Their digestive tract has pockets that stretch down into their legs. They have very high blood pressure, with a heart rate between 90 and 180 beats per minute. Like spiders, they have some additional appendages near their head. One pair is called an “oviger.” The pycnogonid females use this to hand eggs to the males. The males use the ovigers during courtship and to guard the eggs during development. They have pairs of chelifores and palps which are similar to chelicerae and pedipalps found in spiders and are used for feeding and sensing the environment.

Pycnogonids were found in trawling operations during the Challenger Expedition between 1873-1876. I can imagine the confusion these creatures must have caused when they came up in the net. During that voyage they collected more than 4,000 previously unknown species and laid the groundwork for the science of oceanography. And here we are, 130 years later, still asking questions about these bizarre looking creatures.